This invention relates in general to fire detection systems. More specifically, it relates to an automatic fire detection system that processes signals responsive to both infrared (IR) and ultraviolet (UV) radiation in a manner that results in an unusually low incidence of false alarms.
In many situations it is important to monitor an area for fires or incipient explosions. A common example is a facility for the storage or transfer of highly flammable liquid such as liquid propane. Facilities of this type can extend over many acres and include storage tanks, pumping and compressor facilities, and truck loading areas. While most of such a facility is outdoors, portions may be indoors.
An automatic fire detection system for such a facility should respond reliably to any flame, but not trigger an alarm or extinguishers in response to sources of radiation other than a flame. These other sources include sunlight, lightning, welding, and hot objects such as an overheated compressor or the engine of a truck. The quality of the system therefore depends on its ability to discriminate between real flames and non-flame sources of radiation. Response time, sensitivity and range are also important characteristics of the system.
Many known systems respond to the radiation produced by a fire. It is common for such systems to sense IR radiation. For example, U.S. Pat. No. 3,665,440 issued in 1972 to McMenamin relies on the incorrect "fact" that a fire produces IR, but little or no UV. However, it is also known that fires do produce a detectable level of short wave UV radiation. Systems have been produced for many years by the assignee of the present application which detect fires by sensing the presence of short wave UV radiation. While these UV systems are effective and have proven to be commercially successful, they are susceptible to false alarms from non-fire sources of UV such as welding that may occur inside or outside the protected area.
U.S. Pat. Nos. 3,653,016; 3,665,440; 3,825,754; 3,931,521 and 4,199,682 disclose fire or explosion detection systems that employ multiple detection channels, UV detection in conjunction with IR detection, or a combination of these features. In each of these systems, however, the output signal of a detector is characterized by a digital, "yes-no" logic. In systems with multiple channels these digital outputs are applied to conventional logic gates such as AND or NOR gates to produce a resultant output signal that controls an alarm or extinguisher. In particular, the Cinzori '521 patent and the Spector et al. '682 patent apply outputs in excess of preset thresholds to NOR and AND gates respectively so that a fire is signaled when the inputs from both channels carry a positive indication for fire or some other monitored condition. In the Cormier '016 system, the main detector is responsive to visible light and a UV detector is connected in series in the main detector channel. It acts as a simple switch that confirms the presence of a fire. In McMenamin '440 the main detector is responsive to IR, but the system also analyzes the flicker frequency of the IR. Because the flicker frequency is relatively slow, the response time of the system is slow. In addition, McMenamin uses a positive UV output signal in a switch-like manner to inhibit the IR signal. The McMenamin device thus operates on a principle directly contradicted by known UV fire detectors since it assumes that there is little or no UV produced by a flame. It is also significant that the Cinzori '521 and '754 patents use detectors that operate exclusively in the IR spectrum.
While a number of fire detection systems are known, they continue to be susceptible to false alarms, particularly when used outdoors or in an environment where there are non-fire sources of UV such as welding. Known IR detection systems are also characterized by generally poor signal-to-noise ratios and a limited range.
It is therefore a principal object of the present invention to provide an automatic fire detection that reliably and quickly signals the presence of a fire in a protected area while at the same time discriminating sources of IR and UV radiation that are not produced by fire.
A further object of the present invention is to provide a system with the foregoing advantages that automatically compensates for time-varying levels in background IR.
Another object of the invention is to provide a system with the foregoing advantages that is not responsive to transient sources of non-fire radiation.
Yet another object of the invention is to provide a system with the foregoing advantages that is characterized even in outdoor use by excellent sensitivity without complex signal processing electronics and having a long range.
A further object is to provide a system with the foregoing advantages that has a fast response time and can be constructed for a heightened sensitivity to the combustion of a particular type of material.
Another object of this invention is to provide such a system which continuously monitors both IR and UV radiation and can be automatically tested.
A still further object is to provide a single detection system that can signal the presence of a fire, welding or high temperatures in a monitored area.